Process for operating a regenerative fuel cell

ABSTRACT

In a regenerative fuel cell, a positive chamber is separated from a negative chamber by a cation exchange membrane. The positive chamber comprises a positive electrode and a bromine-containing electrolyte. The negative chamber comprises a negative electrode and a sulfide-containing electrolyte. When the cell is in operation, the electrolytes are replenished using electrolyte from an electrolyte store ( 32, 34 ). In the method of the present invention, the distribution of water between the two electrolytes is controlled by discharging the electrolyte(s), such that when the electrolytes are separated by a water permeable membrane ( 46 ), water will flow from the sulfide-containing electrolyte to the bromine-containing electrolyte by osmosis. The present invention involves circulating the electrolytes through a container ( 43 ) which is divided by a water permeable membrane ( 46 ), under conditions wherein no current flows in the container between the two electrolytes.

[0001] The present invention relates to the field of regenerative fuelcell (RFC) technology. In particular it relates to apparatus and methodsfor the operation of RFCs which enhance their performancecharacteristics.

[0002] The manner in which RFCs are able to store and deliverelectricity is well known to those skilled in the art. An example of anRFC is described in U.S. Pat. No. 4,485,154 which discloses anelectrically chargeable, anionically active, reduction-oxidation systemusing a sulfide/polysulfide reaction in one half of the cell and aniodine/iodide, chlorine/chloride or bromine/bromide reaction in theother half of the cell. The two halves of the cell are separated by acation exchange membrane.

[0003] The overall chemical reaction involved, for example, for thebromine/bromide-sulfide/polysulfide system is shown in Equation 1 below:

Br₂+S²⁻⇄2Br⁻+S  Equation 1

[0004] Within an RFC such as that described in U.S. Pat. No. 4,485,154,the reaction takes place in separate but dependent bromine and sulfurhalf-cell reactions as shown below in Equations 2 and 3:

Br₂+2e ⁻⇄2Br⁻  Equation 2

S²⁻⇄2e ⁻+S  Equation 3

[0005] It should be noted however that these equations represent theoverall reactive changes occurring in the RFC. In practice the reactionsare complicated by the low basicity of sulfide which results in theformation of bisulfide as the active species, as shown in Equation 4.

S²⁻+H₂O⇄HS⁻+OH⁻  Equation 4

[0006] Also, the sulfur produced in Equations 1 and 3 forms solublepolysulfide species in the presence of sulfide ions, as shown inEquation 5 (where x may be from 1 to 4).

S²⁻+xS⇄S_(x+1) ²⁻  Equation 5

[0007] Also, free bromine is solubilised in the presence of bromide ionsto form the tribromide ion, as shown in Equation 6

Br⁻+Br₂⇄Br₃ ⁻  Equation 6

[0008] When the RFC is discharging, bromine is converted to bromide onthe positive (+^(ve)) side of the membrane and sulfide is converted topolysulfide on the negative (−^(ve)) side of the membrane. Equation 1goes from left to right and metal ions flow from the −^(ve) side of themembrane to the +^(ve) side of the membrane to complete the circuit.When the RFC is charging, bromide is converted to bromine on the +^(ve)side of the membrane and polysulfide is converted to sulfide on the−^(ve) side of the membrane. Equation 1 goes from right to left andmetal ions flow from the +^(ve) side of the membrane to the −^(ve) sideof the membrane to complete the circuit.

[0009] The discharge/charge cycle described above will be repeated manytimes during the lifetime of the RFC and in order for the RFC to workefficiently throughout its lifetime it is important that thedistribution of water (which is the solvent for the reactions describedabove) between the two electrolytes is controlled.

[0010] In a RFC such as that described above there will inevitably be ashift of water molecules from the +^(ve) side of the membrane to the−^(ve) side of the membrane during the charge cycle. This is because themetal ions which flow from the +^(ve) side to the −^(ve) side during thecharge cycle carry with them a salvation sphere made up of watermolecules. Similarly, there will inevitably be a shift of watermolecules from the −^(ve) side of the membrane to the +^(ve) side of themembrane during the discharge cycle as the metal ions and theirsalvation sphere flow back across the membrane. Thus, although waterinevitably shifts backwards and forwards across the membrane during thecharge/discharge cycle, in an ideal system there is no net change in thedistribution of water after a complete cycle.

[0011] At the beginning of the RFC's lifetime the two electrolytes willbe set up with the distribution of water between them fixed at thedesired amount. The water distribution will usually be fixed such that,at the halfway point in the charge (or discharge) cycle, theelectrolytes are osmotically balanced. Unfortunately, once the RFCbegins to operate in its repeating discharge-charge cycle, factors mayintervene which result in the occurrence of coulombic losses. That is tosay, not all of the charge delivered to the RFC during the charge cycleis recovered during the discharge cycle. One of the results of suchcoulombic losses is that the water distribution between the electrolytescan become unbalanced, as explained below.

[0012] The factors which result in coulombic losses in the system willvary depending upon the identity of the reactive species within theelectrolytes and on the manner in which the RFC is constructed andoperated.

[0013] In the case of the bromine/bromide-sulfide/polysulfide RFC suchas that described above, an important factor which results in coulombicloss is the diffusion of unwanted species across the membrane. Althougha cation selective ion-exchange membrane is used, 100% permselectivityis not possible and during extended cycling of the cell some anionicspecies diffuse through the membrane. In particular, sulfide ions(largely present in the bisulfide form, HS⁻) and polysulfide ions(S_(x+1) ²⁻, where x may be from 1 to 4) may diffuse from thesulfide/polysulfide electrolyte into the bromine/bromide electrolytewhere they will be oxidised by the bromine to form sulfate ions as shownin equations 7 and 8 below:

HS⁻+4Br₂+4H₂O→8Br⁻+SO₄ ²⁻+9H⁺  Equation 7

S_(x+1) ²⁻+(3x+4)Br₂+(4x+4)H₂O→(6x+8)Br⁻+(x+1)SO₄ ²⁻+(8x+8)H⁺  Equation8

[0014] Imperfections other than diffusion through the membrane whichcould similarly contribute to the above process are ineffective sealingbetween cell compartments, or catastrophic failure of any of the cellseparating components, each of which may result in crossover of theelectrolytes between cell compartments.

[0015] In Equations 7 and 8, the oxidation of the sulfur species goesbeyond that which occurs during normal operation of the RFC. That is tosay, the sulfide and polysulfide ions are oxidised all the way tosulfate ions. Consequently, in the case of sulfide ion cross-over(Equation 7), four bromine molecules per sulfide ion are consumed ratherthan the one bromine molecule per sulfide ion which is normally consumedin the reaction scheme of Equation 1. Similar overconsumption of bromineresults from polysulfide cross-over (Equation 8) although to a slightlylesser extent. As a result, the bromine/bromide electrolyte becomesdischarged to a greater extent than the sulfide/polysulfide electrolyte.Thus, when the cell is discharging there is insufficient bromine presentto react with all the sulfide ions present thereby preventing completionof the discharge cycle. As a result, the voltage generated by the cellbegins to decline earlier in the discharge cycle than when theelectrolytes are balanced. In effect, the reactions represented byEquations 7 and 8 result in the permanent conversion of some of thepolysulfide ions to sulfide because not all of the polysulfide ions arerecovered on discharge. Subsequent cycles repeat this process, furtherreducing the number of polysulfide ions present.

[0016] Another feature of RFCs which commonly leads to the occurrence ofcoulombic losses is the presence of shunt currents. These arise when theRFC is made up of many cells connected in series and the twoelectrolytes are pumped in parallel through the respective chambers ofeach cell. The potential difference which exists between the downstreamelectrolyte and the upstream electrolyte can result in the flow ofcharge through the pathway of a single electrolyte rather than acrossthe membrane from one electrolyte to the other.

[0017] The net result of these processes is that, at a given current,the time required to completely charge the the RFC becomes greater thanthe time taken for it to discharge. Since the current is carried by themetal ions (and their accompanying solvation spheres) there areconsequently more water molecules transferred from the bromine/bromideelectrolyte to the sulfide/polysulfide electrolyte during the chargecycle than are returned back during the discharge cycle. That is to say,there is a net shift of water from the bromine/bromide electrolyte tothe sulfide/polysulfide electrolyte. Whilst the water shift may benegligible for a single cycle, RFCs typically operate continuously formany cycles and this can result in a large water shift which presentssignificant problems in handling the electrolytes.

[0018] It would therefore be advantageous to provide a process forcontrolling the water shift such that the distribution of water betweenthe electrolytes is maintained at, or returned to, its preferred state.

[0019] Accordingly, the present invention provides an electrochemicalprocess for energy storage and/or power delivery comprising:

[0020] (i) maintaining and circulating electrolyte flows in a liquidsystem in which the active constituents are soluble in a single cell orin an array of repeating cell structures, each cell with a positive(+^(ve)) chamber containing a +^(ve) electrode and a negative (−^(ve))chamber containing a −^(ve) electrode, the chambers being separated fromone another by a cation exchange membrane, the electrolyte circulatingin the −^(ve) chamber of each cell during discharge containing a sulfide(electrolyte 1), and the electrolyte circulating in the +^(ve) chamberduring discharge containing bromine (electrolyte 2),

[0021] (ii) restoring or replenishing the electrolytes in the +^(ve) and−^(ve) chambers by circulating the electrolyte from each chamber tostorage means comprising a volume of electrolyte greater than the cellvolume for extended delivery of power over a longer discharge cycle thanthe cell volume alone would permit, and

[0022] (iii)controlling the distribution of water between the twoelectrolytes by a process comprising (a) discharging electrolytes 1and/or 2, or fractions thereof, to such an extent that water will flowby osmosis from electrolyte 1 into electrolyte 2 when said electrolytesare separated by a water permeable membrane and (b) circulatingelectrolytes 1 and 2, or fractions thereof, through the first and secondchambers respectively of a container which is divided by a waterpermeable membrane, under conditions wherein no current flows in saidcontainer between electrolytes 1 and 2.

[0023] The present invention also includes within its scope apparatusfor carrying out a process as described above comprising:

[0024] (i) a single cell or an array of repeating cell structures, eachcell comprising; a +^(ve) chamber containing a +^(ve) electrode and a−^(ve) chamber containing a −^(ve) electrode the chambers beingseparated from one another by an ion exchange membrane, an electrolytecirculating in the −^(ve) chamber of each cell which contains a sulfideduring discharge (electrolyte 1), and an electrolyte circulating in the+^(ve) chamber which contains bromine during discharge (electrolyte 2),

[0025] (ii) storage and circulation means for each electrolyte forrestoring or replenishing the electrolytes in the +^(ve) and −^(ve)chambers,

[0026] (iii)means for controlling the distribution of water between thetwo electrolytes comprising; means for discharging electrolytes 1 and/or2, or fractions thereof, to such an extent that water will flow byosmosis from electrolyte 1 into electrolyte 2 when said electrolytes areseparated by a water permeable membrane; a container which is dividedinto first and second chambers by a water permeable membrane; and meansfor circulating electrolytes 1 and 2, or fractions thereof, respectivelythrough the first and second chambers of the container under conditionswherein no current flows in said container between electrolytes 1 and 2.

[0027] Upon circulation of the electrolytes through the chambers of thedivided container, water will flow by osmosis from electrolyte 2 (thesulfide/polysulfide electrolyte) through the water permeable membraneinto electrolyte 1 (the bromine/bromide electrolyte) thus returning thewater which has shifted across the cation exchange membrane duringextended cycling of the RFC.

[0028] It is necessary to discharge one or both of electrolytes 1 and 2prior to effecting the osmotic redistribution of water becausedischarging electrolyte 2 increases its ionic strength (bromine isconverted to bromide) and discharging electrolyte 1 decreases its ionicstrength (sulfide is converted to polysulfide). This creates thedifference in ionic strength required to ensure that water flows from 1to 2 and not vice versa. In a preferred embodiment, electrolyte 1 iscompletely discharged so as to provide the lowest possible ionicstrength for electrolyte 1. In another preferred embodiment, electrolyte2 is completely discharged so as to provide the highest possible ionicstrength for electrolyte 2. In a particularly preferred embodiment, bothelectrolytes 1 and 2 are completely discharged so as to provide thegreatest possible difference in ionic strength between electrolytes 1and 2, thereby maximising the rate of flow of water from 1 to 2 andimproving the efficiency of the redistribution process.

[0029] It is necessary to carry out step (iii) (b) of the process underconditions wherein no current flows between electrolytes 1 and 2 as theypass through the container so that the ionic strength of saidelectrolytes does not vary during this step (for reasons other than thedesired redistribution of water). It should be noted that this does notnecessarily mean that current may not flow between electrolytes 1 and 2in other parts of the RFC system during the process of the presentinvention.

[0030] Suitable water permeable membranes are those which permit thetransfer by osmosis of water from electrolyte 1 to electrolyte 2 butwhich substantially prevent the transfer of the reactive ingredients ofthe electrolytes (i.e. bromine/bromide and/or sulfide/polysulfide). Thewater permeable membrane of step (iii) may be the same as the cationexchange membrane used in the cells described in step (i). However,since no current flows across the water permeable membrane during step(iii) (b) it is possible to use a different membrane structure withcharacteristics more suited to the transfer of water than the transferof cations. Examples of suitable water permeable membranes include, butare not limited to membranes commonly used in reverse osmosis,nanofiltration and cation exchange.

[0031] Whilst the process of the present invention may be carried out onthe entire volumes of the electrolytes, this reduces the operationalavailability of the RFC due to the necessity of conducting the processunder conditions of zero current flow. It is therefore preferable forstep (iii)(b), or steps (iii)(a) and (iii)(b) of the process to becarried out on fractions of the total volume of each of the electrolyteswhich are removed from, and subsequently returned to, the main streamsof electrolytes circulating throughout the RFC.

[0032] In this regard, step (iii) of the process may be applied as abatch process to a RFC operating in accordance with steps (i) and (ii).Thus, fractions of the bromine/bromide electrolyte (electrolyte 1) andsulfide/polysulfide electrolyte (electrolyte 2) are removed from the RFCmain streams when sufficiently discharged. Thus, step (iii)(a) occurswithin the RFC itself as a part of its normal discharge cycle.Electrolytes 1 and 2 are then treated in separate apparatus suitable forcarrying out step (iii) (b) of the process before being returned to theRFC.

[0033] Thus, in one embodiment, fractions of electrolytes 1 and 2 areremoved from the main streams of the RFC at, or close to, the end of thedischarge cycle. These two fractions, which may be stored in separatetanks, are then circulated through the first and second chambersrespectively of a container which is divided by a water permeablemembrane, under conditions wherein no current flows betweenelectrolytes. When the desired amount of water transfer from electrolyte1 to electrolyte 2 has taken place, the fractions can be returned totheir respective main streams within the RFC.

[0034] In this embodiment, with regard to the apparatus described aboveas suitable for carrying out the process of the present invention, the“means for discharging electrolytes 1 and/or 2” is the RFC itself.

[0035] Alternatively, the process may be applied continuously to the RFCwherein side streams of the bromine/bromide and sulfide/polysulfideelectrolytes drawn continuously from the main streams of the RFC arediverted through apparatus suitable for carrying out both steps (iii)(a)and (iii)(b) of the process before being returned to the RFC.

[0036] Thus, in one embodiment, fractions of electrolytes 1 and 2 areremoved from the main streams of the RFC at any point in thecharge/discharge cycle. One or both fractions of electrolytes 1 and 2are then discharged to a sufficient extent by means of an auxiliaryelectrochemical cell and are then circulated through the first andsecond chambers respectively of a container which is divided by a waterpermeable membrane, under conditions wherein no current flows betweenthe electrolytes. When the desired amount of water transfer fromelectrolyte 1 to electrolyte 2 has taken place, the fractions can bereturned to their respective main streams.

[0037] In this embodiment, with regard to the apparatus described aboveas suitable for carrying out the process of the present invention, the“means for discharging electrolytes 1 and/or 2” is provided by way of anauxiliary electrochemical cell.

[0038] The process and apparatus of the present invention may beadvantageously combined with the processes and apparatus described inthe applicants patent application nos PCT/GB99/02103 (published asWO00/03448) and/or PCT/GB00/02536 (published as WO01/03221) so as toprovide a complete electrolyte management system. Suitable embodimentsof such a system are illustrated in the figures and described in detailbelow.

[0039] In another aspect, the present invention also provides for theuse, in a process for energy storage and/or power delivery comprising:

[0040] (i) maintaining and circulating electrolyte flows in a liquidsystem in which the active constituents are soluble in a single cell orin an array of repeating cell structures, each cell with a positive(+^(ve)) chamber containing a +^(ve) electrode and a negative (−^(ve))chamber containing a −^(ve) electrode, the chambers being separated fromone another by a cation exchange membrane, the electrolyte circulatingin the −^(ve) chamber of each cell during discharge containing a sulfide(electrolyte 1), and the electrolyte circulating in the +^(ve) chamberduring discharge containing bromine (electrolyte 2), and

[0041] (ii) restoring or replenishing the electrolytes in the +^(ve) and−^(ve) chambers by circulating the electrolyte from each chamber tostorage means comprising a volume of electrolyte greater than the cellvolume for extended delivery of power over a longer discharge cycle thanthe cell volume alone would permit;

[0042] of a process comprising:

[0043] (a) discharging electrolytes 1 and/or 2, or fractions thereof, tosuch an extent that water will flow by osmosis from electrolyte 1 intoelectrolyte 2 when said electrolytes are separated by a water permeablemembrane, and

[0044] (b) circulating the discharged electrolytes 1 and 2, or fractionsthereof, through the first and second chambers respectively of acontainer which is divided by a water permeable membrane, underconditions wherein no current flows in said container betweenelectrolytes 1 and 2;

[0045] for the purpose of controlling the distribution of water betweenthe two electrolytes.

[0046] The present invention will be further described with reference tothe accompanying drawings in which:

[0047]FIG. 1A is a schematic view of a basic electrochemicalreduction-oxidation cell in which a sulfide/polysulfide reaction iscarried out in one half of the cell and a bromine/bromide reaction iscarried out in the other half of the cell;

[0048]FIG. 1B is a diagram of cell arrays using the system of FIG. 1A;

[0049]FIG. 2 is a flow diagram of a fluid flow system using the cell ofFIG. 1A or cell array of FIG. 1B;

[0050]FIG. 3 is a flow diagram of an apparatus for carrying out theprocess of the present invention as a batch process.

[0051]FIG. 4 is a flow diagram of an apparatus for carrying out theprocess of the present invention as a continuous process.

[0052]FIG. 5 is a flow diagram of an apparatus for carrying out theprocess of the present invention as a continuous process.

[0053]FIG. 6 is a flow diagram of an apparatus for carrying out theprocess of the present invention as a continuous process.

[0054]FIG. 7 is a flow diagram of an apparatus for carrying out theprocess of the present invention as a continuous process, includingmeans for effecting sulfate removal.

[0055]FIG. 8 is a flow diagram of an apparatus for carrying out theprocess of the present invention as a continuous process, includingmeans for effecting sulfate removal and means for rebalancing theelectrolytes.

[0056]FIG. 9 is a graph showing the change in electrolyte volumes duringnormal operation of a bromine/bromide-sulfide/polysulfide RFC and whenthe process of the present invention is used.

[0057]FIG. 10 is a graph showing the change in volume of thesufide/polysulfide electrolyte during normal operation of abromine/bromide-sulfide/polysulfide RFC and when the process of thepresent invention is used.

[0058]FIG. 1A shows a cell 10 with a positive (+^(ve)) electrode 12 anda negative (−^(ve)) electrode 14 and a cation exchange membrane 16 whichmay be formed from a fluorocarbon polymer with sulfonic acid functionalgroups to provide charge carriers. The membrane 16 acts to separate the+^(ve) and −^(ve) sides of the cell 10 and is selected to minimizemigration of bromine from the +^(ve) side to the −^(ve) side and tominimize migration of sulfide and polysulfide ions from the −^(ve) sideto the +^(ve) side. An aqueous solution 22 of NaBr is provided in achamber 22C formed between the +^(ve) electrode 12 and the membrane 16and an aqueous solution 24 of Na₂S_(x), (where x may be from 2 to 5) isprovided in a chamber 24C formed between the −^(ve) electrode 14 and themembrane 16. A K₂S_(x) solution, which is more soluble and moreexpensive than the Na₂S_(x) solutions, is used in another embodiment.

[0059] When the cell is in the discharged state, a solution of NaBr ofup to 6.0 molar concentration exists in the chamber 22C of the cell anda solution of Na₂S_(x) at 0.5 to 1.5 molar, exists in chamber 24C of thecell. Higher molarity is possible with K₂S_(x).

[0060] As the cell is charged, Na⁺ions are transported through thecation membrane 16, as shown in FIG. 1A, from the +^(ve) to the −^(ve)side of the cell. Free bromine is produced via oxidation of the bromideions at the +^(ve) electrode and dissolves as a tribromide orpentabromide ion. Sulfur is reduced at the −^(ve) electrode and thepolysulfide, Na₂S_(x), salt eventually becomes the monosulfide as thecharging proceeds to completion. At the +^(ve) side the followingreaction occurs,

2Br⁻⇄Br₂+2e⁻

[0061] and at the −^(ve) side the following reaction occurs,

S+2e⁻⇄S²⁻.

[0062] The membrane separates the two electrolytes and prevents bulkmixing and also retards the migration of sulfide and polysulfide ionsfrom the −^(ve) side to the +^(ve) side, and the migration of Br⁻and Br₂from the +^(ve) to the −^(ve) side. Diffusion of the sulfide andpolysulfide ions across the membrane results in coulombic losses andwater shift as described earlier.

[0063] When providing power, the cell is discharging. During thisaction, reversible reactions occur at the two electrodes. At the +^(ve)side electrode 12, bromine is reduced to Br⁻, and at the −^(ve)electrode, the S²⁻ion is oxidized to molecular S. The electrons producedat the −^(ve) electrode form the current through a load. The chemicalreaction at the +^(ve) electrode produces 1.06 to 1.09 volts and thechemical reaction at the −^(ve) electrode produces 0.48 to 0.52 volts.The combined chemical reactions produce an open circuit voltage of 1.54to 1.61 volts per cell.

[0064] The present system is an anionically active electrochemicalsystem. Therefore, the cation which is associated with them essentiallytakes no part in the energy producing process. Hence, a cation of“convenience” is chosen. Sodium or potassium are preferred choices.Sodium and potassium compounds are plentiful, they are inexpensive andhave high water solubilities. Lithium and ammonium salts are alsopossibilities, but at higher costs.

[0065]FIG. 1B shows an array 20 of multiple cells connected inelectrical series and fluid parallel. Multiple mid-electrodes 13 (eachone having a +^(ve) electrode side 12A and −^(ve) electrode side 14A)and end electrodes 12E (+^(ve)) and 14E (−^(ve)) are spaced out fromeach other by membranes 16 and, optionally, screen or mesh spacers (22D,24D) in all the cell chambers 22C, 24C, (portions of two of which 22D,24D are shown by way of example) to form end cells C_(E1), and C_(E2)and an array of mid cells C_(M) (typically 10-20; but note much smallerand much higher numbers of cells can be accommodated). The endelectrodes 12E (+^(ve)) and 14E (−^(ve)) have internal conductors 12Fand 14F (typically copper screens) encapsulated therein and leading toexternal terminals 12G, 14G which are connected to external loads (e.g.to motor(s) via a control circuit (CONT), the motor(s) may be used todrive a vehicle) or power sources (e.g. utility power grid when used asa load-levelling device).

[0066]FIG. 2 shows a free flow system, a power generation/storage systemutilizing one or more of the batteries or cell array formats 20. Eachcell 20 receives electrolyte through pumps 26 and 28 for the NaBr andNa₂S_(x) solutions (22 and 24, respectively). The electrolytes 22 and 24are stored in containers 32 and 34. The tanks 32, 34 can be replacedwith freshly charged electrolyte by substituting tanks containing freshelectrolyte and/or refilling them from charged supply sources via lines32R, 34R with corresponding lines (not shown) provided for drainingspent (discharged) reagent. The electrolytes 22 and 24 are pumped fromtanks 32 and 34, respectively, into the respective chambers 22C and 24Cby means of pumps 26 and 28.

[0067]FIG. 3 shows a free flow system comprising a powergeneration/storage system as illustrated in FIG. 2 and a water balancesystem 40. Fractions of the sulfide/polysulfide electrolyte (electrolyte1) and the bromine/bromide electrolyte (electrolyte 2) are taken fromtanks 32 and 34 respectively at a point in the charge/discharge cyclewhen the electrolytes are sufficiently discharged, preferably at or nearthe end of the discharge cycle. The electrolyte fractions are passed totanks 41 and 42. Electrolytes 1 and 2 are then circulated throughchambers 44 and 45 respectively of a container 43 which is divided by awater permeable membrane 46. It will be appreciated by a person skilledin the art the an array of containers 43 arranged in series mayadvantageously be used in this and the other embodiments describedherein. The relative difference in ionic strength between theelectrolytes means that water will flow by osmosis through the waterpermeable membrane from electrolyte 1 into electrolyte 2. Whensufficient water transfer has occurred the electrolytes 1 and 2 arereturned to tanks 32 and 34 respectively. This system can only be usedin a batch process because electrolytes 1 and 2 can only be removed whenthey are sufficiently discharged.

[0068]FIG. 4 shows an alternative free flow system comprising a powergeneration/storage system as illustrated in FIG. 2 and an alternativewater balance system 50. This embodiment differs from FIG. 3 in that thewater balance system 50 comprises a container 53 which also functions asan auxiliary electrochemical cell. In this embodiment fractions of thesulfide/polysulfide electrolyte (electrolyte 1) and the bromine/bromideelectrolyte (electrolyte 2) are taken from tanks 32 and 34 respectivelyat any point in the charge/discharge cycle, but still preferably at ornear the end of the discharge cycle. The electrolytes are passed totanks 51 and 52. Electrolytes 1 and 2 are then circulated throughchambers 54 and 55 respectively of the container 53 which is divided bya water permeable membrane 56. It will be appreciated by a personskilled in the art the an array of containers 53 arranged in series mayadvantageously be used in this and the other embodiments describedherein. In the present embodiment the water permeable membrane 56 isalso a cation exchange membrane (for example like membrane 16 used inthe main RFC system) and the container additionally comprises electrodes57 and 58 so that the container may function as a cell in the samemanner as the cell (or cells) of the main RFC system. Current is passedthrough the container so as to completely discharge electrolytes 1 and 2as they circulate through chambers 54 and 55. The current is thenswitched off and circulation of the electrolytes continues. The relativedifference in ionic strength between the electrolytes means that waterwill flow by osmosis through the water permeable membrane fromelectrolyte 1 into electrolyte 2. When sufficient water transfer hasoccurred the electrolytes 1 and 2 are returned to tanks 32 and 34respectively. This system can be used in a batch or continuous processbecause electrolytes 1 and 2 can be removed from the main system at anypoint in the charge/discharge.

[0069]FIG. 5 shows an alternative free flow system which is essentiallya combination of FIGS. 3 and 4. In this embodiment, fractions of thesulfide/polysulfide electrolyte (electrolyte 1) and the bromine/bromideelectrolyte (electrolyte 2) are taken from tanks 32 and 34 respectivelyat any point in the charge/discharge cycle, but still preferably at ornear the end of the discharge cycle. The electrolytes are passed totanks 51 and 52. Electrolytes 1 and 2 are then circulated throughchambers 54 and 55 respectively of a container 53 which is divided by awater permeable membrane 56 which is also a cation exchange membrane.The container additionally comprises electrodes 57 and 58 so that thecontainer may function as a cell in the same manner as the cell (orcells) of the main RFC system. Current is passed through thecontainer/cell so as to completely discharge electrolytes 1 and 2 asthey circulate through chambers 54 and 55. Rather than switch thecurrent off and continue circulation within system 50 (as occurs in theembodiment of FIG. 4), electrolytes 1 and 2 are passed to tanks 41 and42 respectively. Electrolytes 1 and 2 are then circulated throughchambers 44 and 45 respectively of a container 43 which is divided by awater permeable membrane 46. The relative difference in ionic strengthbetween the electrolytes means that water will flow by osmosis throughthe water permeable membrane from electrolyte 1 into electrolyte 2. Whensufficient water transfer has occurred the electrolytes 1 and 2 arereturned to tanks 32 and 34 respectively. This embodiment can also beused in a batch or continuous process because electrolytes 1 and 2 canbe removed from the main system at any point in the charge/discharge. Ithas an advantage over the embodiment of FIG. 3 in that it is notnecessary to interrupt the flow of current through the auxiliaryelectrochemical cell 53 in order to allow water transfer. Water transferoccurs in a separate system 40 and the water permeable membrane 46 canbe selected according the its ability to effect water transfer withoutthe constraints of having to function as a cation exchange membrane aswell.

[0070]FIG. 6 shows an alternative free flow system which operates on asimilar principle to the embodiment of FIG. 5. In this case only thebromine/bromide electrolyte is discharged in order to create thenecessary difference in ionic strength between the electrolytes. As inFIG. 5, an auxiliary electrochemical cell is again used to effect thedischarge. In this embodiment fractions of the bromine/bromideelectrolyte (electrolyte 2) are taken from tank 34 at any point in thecharge/discharge cycle, but still preferably at or near the end of thedischarge cycle. The electrolyte is passed to tanks 51 and 52.Electrolyte 2 is then circulated through chambers 54 and 55 respectivelyof a container 53 which is divided by a cation exchange membrane 56. Thecontainer additionally comprises electrodes 57 and 58 so that thecontainer functions as a cell. Current is passed through the containerso as to discharge electrolyte 2 as it circulates through chamber 54 andto charge electrolyte 2 as it circulates through chamber 55. It will beapparant to a person skilled in the art that oxidisable electrolytesother than electrolyte 2 could be circulated from tank 52 throughchamber 55 of container/cell 53 in order to effect discharge ofelectrolyte 2 which circulates through chamber 54. However, it ispreferred to use a fraction of electrolyte 2 taken from tank 34 becausethis results in said fraction being charged to some degree. Electrolyte2 which circulated through chamber 55 is returned to tank 34 whilstelectrolyte 2 which circulated through chamber 54 is passed to tank 42of the water balance system 40. Electrolyte 2 is then circulated throughchamber 45 of container 43 which is divided by a water permeablemembrane 46. Electrolyte 1 taken from tank 32 of the main system istransferred to holding tank 41 and from there circulated through chamber44 before being returned to tank 32. The relative difference in ionicstrength between the electrolytes means that water will flow by osmosisthrough the water permeable membrane from electrolyte 1 into electrolyte2. When sufficient water transfer has occurred the electrolyte 2 isreturned to tank 34. This embodiment can also be used in a batch orcontinuous process because electrolytes 1 and 2 can be removed from themain system at any point in the charge/discharge.

[0071]FIG. 7 shows an embodiment which works in the same manner as theembodiment of FIG. 6. In addition, between tanks 51 and 42 there isincluded a sulfate removal system 60 which comprises a crystalliser 61and a filter 62. This embodiment therefore combines the water balanceadvantages of the present invention with the advantages of the processfor the removal of sulfate ions from electrolyte 1 which is described inthe applicants International Patent Application No GB99/02103 (publishedas WO00/03448).

[0072]FIG. 8 shows an embodiment which works in the same manner as theembodiment of FIG. 7. In addition, between the auxiliary electrochemicalcell 50 and the sulfate removal system 60 there is included a furtherauxiliary cell 70. Electrolyte 2 from tank 51 passes to tank 71 fromwhich it is circulated through chamber 74 of container 73 which isdivided by a cation exchange membrane 76. The electrolyte circulatingthrough chamber 75 is a fraction of electrolyte 2 taken from tank 34 viaa three way distributer 81 and stored in tank 72. When a current isapplied to the cell via electrodes 77 and 78, bromide ions contained inelectrolyte 2 circulating through chamber 75 are oxidised to bromine.This charges the fraction of electrolyte 2 taken from tank 34 and helpsto rebalance the state of charge of the electrolytes of the RFC systemas explained in the applicants International Patent Application NoGB99/02103 (published as WO01/03221). Electrolyte 2 which circulatesthrough chamber 74 is however free from bromine molecules due to thedischarging process effected by auxiliary electrochemical cell 50. Thusthe next reducible species present in the electrolyte is water which isreduced to hydrogen gas and hydroxide ions. Electrolyte 2 passes fromchamber 74 to tank 79 from which hydrogen gas may be vented prior toreturning electrolyte 2 to tank 71. From tank 71, electrolyte 2 passesthrough the sulfate removal system 60 and onto the water balance system40 prior to returning to tank 34 of the main RFC system via a headertank 80 which collects electrolyte 2 from tanks 42, 52 and 72. Theinclusion of the further auxiliary cell is not only advantageous due tothe benificial effect it has on rebalancing the state of charge of theelectrolytes. It is also beneficial because it results in an increase inthe ionic strength of the fraction of electrolyte 2 which passes throughchamber 74. This promotes crytallisation of sulfate salts in the sulfateremoval system 60 and also promotes the transfer of water in the waterbalance system 40. This embodiment therefore combines (and enhances) thewater balance advantages of the present invention with the advantages ofthe process for the removal of sulfate ions from electrolyte 1 which isdescribed in the applicants International Patent Application NoGB99/02103 (published as WO00/03448) and also with the advantages of theelectrolyte rebalancing system which is described in the applicantsInternational Patent Application No GB99/02103 (published asWO01/03221). It will be appreciated that the presence of the sulfateremoval system 60 is not essential to the invention and a furtherembodiment equivalent to that illustrated in FIG. 8 but excluding suchsystem would also be advantageous.

[0073] It will be appreciated by the person skilled in the art thatfurther embodiments may be described and the present invention is notintended to be limited to those described herein.

EXAMPLE 1

[0074] A regenerative fuel cell of the type described above withreference to FIGS. 1A, 1B and 2 having sulfide/polysulfide andbromine/bromide electrolytes was set up. The starting composition of thesulfide/polysulfide electrolyte was Na₂S_(3.7) (1.3M), NaOH (1M) andNaBr (1M). The starting composition of the bromine/bromide electrolytewas NaBr (5M). The cell had the following specifications: Electrodematerial: polyethylene impregnated with activated carbon. Electrodearea: 140 m². Current density: 60 mA/cm². Electrolyte volume: 23,000litres per electrolyte Cycle time: 4.5 hrs Flow rate: 136 l/hr Membranematerial: Nafion ™ 115

[0075] The cell was operated for 20 cycles and the electrolyte tanklevels were monitored with time. The discharged electrolytes were thenpumped through the module under conditions of zero current flow. Theresults are illustrated in FIG. 9 which shows that during normaloperation of the cell there is a net increase in the polysulfide tanklevels and a net decrease in the bromine tank levels. This net changecan be clearly seen despite the variations in the tank levels duringeach cycle. When the electrolytes are discharged and the electrolytesare pumped under conditions of zero current flow the tank levels can beseen to return to their original levels.

EXAMPLE 2

[0076] A regenerative fuel cell of the type described above withreference to FIGS. 1A, 1B and 2 having sulfide/polysulfide andbromine/bromide electrolytes was set up. The starting composition of thesulfide/polysulfide electrolyte was Na₂S_(3.7) (1.3M), NaOH (1M) andNaBr (1M). The starting composition of the bromine/bromide electrolytewas NaBr (5M). The cell had the following specifications: Electrodematerial: polyethylene impregnated with activated carbon. Electrodearea: 5.5 m². Current density: 60 mA/cm². Electrolyte volume: 10 litresper electrolyte Cycle time: 3 hrs Flow rate: 4 l/hr Membrane material:Nafion ™ 115

[0077] The cell was operated for approximately 11 cycles (32 to 33hours). The discharged electrolytes were then pumped through the moduleunder conditions of zero current flow for about 5 hours. The cell wasonce again operated for approximately 11 cycles (32 to 33 hours). Thedischarged electrolytes were then pumped through the module underconditions of zero current flow for about 72 hours. The polysulfideelectrolyte tank level was monitored throughout. The results areillustrated in FIG. 10 which shows that during normal operation of thecell there is an increase in the polysulfide tank level. When theelectrolytes are discharged and the electrolytes are pumped underconditions of zero current flow the polysulfide electrolyte tank levelcan be seen to decrease towards (and even below) its original level.

1. An electrochemical process for energy storage and/or power deliverycomprising: (i) maintaining and circulating electrolyte flows in aliquid system in which the active constituents are soluble in a singlecell or in an array of repeating cell structures, each cell with apositive (+^(ve)) chamber containing a +^(ve) electrode and a negative(−^(ve)) chamber containing a −^(ve) electrode, the chambers beingseparated from one another by a cation exchange membrane, theelectrolyte circulating in the −^(ve) chamber of each cell duringdischarge containing a sulfide (electrolyte 1), and the electrolytecirculating in the +^(ve) chamber during discharge containing bromine(electrolyte 2), (ii) restoring or replenishing the electrolytes in the+^(ve) and −^(ve) chambers by circulating the electrolyte from eachchamber to storage means comprising a volume of electrolyte greater thanthe cell volume for extended delivery of power over a longer dischargecycle than the cell volume alone would permit, and (iii)controlling thedistribution of water between the two electrolytes by a processcomprising (a) discharging electrolytes 1 and/or 2, or fractionsthereof, to such an extent that water will flow by osmosis fromelectrolyte 1 into electrolyte 2 when said electrolytes are separated bya water permeable membrane and (b) circulating electrolytes 1 and 2, orfractions thereof, through the first and second chambers respectively ofa container which is divided by a water permeable membrane, underconditions wherein no current flows in said container betweenelectrolytes 1 and
 2. 2. A process as claimed in claim 1 wherein thewater permeable membrane is a reverse osmosis membrane or ananofiltration membrane or a cation exchange membrane.
 3. A process asclaimed in claim 1 or claim 2 wherein step (iii)(b) of the process iscarried out on a fraction of the total volume of electrolytes 1 and 2.4. A process as claimed in claim 3 wherein step (iii)(a) of the processis carried out on a fraction of the total volume of electrolytes 1and/or
 2. 5. A process as claimed in claim 3 wherein fractions ofelectrolytes 1 and 2 are removed from the electrolyte flows at, or closeto, the end of the discharge cycle and circulated through the first andsecond chambers respectively of a container which is divided by a waterpermeable membrane, under conditions wherein no current flows betweenelectrolytes, the fractions being returned to the electrolyte flows oncethe desired amount of water transfer from electrolyte 1 to electrolyte 2has taken place.
 6. A process as claimed in claim 4 wherein fractions ofelectrolytes 1 and 2 are removed from the electrolyte flows at any pointin the charge/discharge cycle and one or both electrolytes are thendischarged to a sufficient extent by means of an auxiliaryelectrochemical cell and are then circulated through the first andsecond chambers respectively of a container which is divided by a waterpermeable membrane, under conditions wherein no current flows betweenthe electrolytes, the fractions being returned to the electrolyte flowsonce the desired amount of water transfer from electrolyte 1 toelectrolyte 2 has taken place.
 7. Apparatus for carrying out a processas claimed in claims 1 to 6 comprising: (i) a single cell or an array ofrepeating cell structures, each cell comprising; a +^(ve) chambercontaining a +^(ve) electrode and a −^(ve) chamber containing a −^(ve)electrode the chambers being separated from one another by an ionexchange membrane, an electrolyte circulating in the −^(ve) chamber ofeach cell which contains a sulfide during discharge (electrolyte 1), andan electrolyte circulating in the +^(ve) chamber which contains bromineduring discharge (electrolyte 2), (ii) storage and circulation means foreach electrolyte for restoring or replenishing the electrolytes in the+^(ve) and −^(ve) chambers, (iii)means for controlling the distributionof water between the two electrolytes comprising; means for dischargingelectrolytes 1 and/or 2, or fractions thereof, to such an extent thatwater will flow by osmosis from electrolyte 1 into electrolyte 2 whensaid electrolytes are separated by a water permeable membrane; acontainer which is divided into first and second chambers by a waterpermeable membrane; and means for circulating electrolytes 1 and 2, orfractions thereof, respectively through the first and second chambers ofthe container under conditions wherein no current flows in saidcontainer between electrolytes 1 and
 2. 8. Apparatus as claimed in claim7 wherein the means for discharging electrolytes 1 and/or 2, orfractions thereof, to such an extent that water will flow by osmosisfrom electrolyte 1 into electrolyte 2 when said electrolytes areseparated by a water permeable membrane is provided by the single cellor array of repeating cell structures which forms part (i) of theapparatus.
 9. Apparatus as claimed in claim 7 wherein the means fordischarging electrolytes 1 and/or 2, or fractions thereof, to such anextent that water will flow by osmosis from electrolyte 1 intoelectrolyte 2 when said electrolytes are separated by a water permeablemembrane is provided by way of an auxiliary electrochemical cell. 10.Use, in a process for energy storage and/or power delivery comprising:(i) maintaining and circulating electrolyte flows in a liquid system inwhich the active constituents are soluble in a single cell or in anarray of repeating cell structures, each cell with a positive (+^(ve))chamber containing a +^(ve) electrode and a negative (−^(ve)) chambercontaining a −^(ve) electrode, the chambers being separated from oneanother by a cation exchange membrane, the electrolyte circulating inthe −^(ve) chamber of each cell during discharge containing a sulfide(electrolyte 1), and the electrolyte circulating in the +^(ve) chamberduring discharge containing bromine (electrolyte 2), and (ii) restoringor replenishing the electrolytes in the +^(ve) and −^(ve) chambers bycirculating the electrolyte from each chamber to storage meanscomprising a volume of electrolyte greater than the cell volume forextended delivery of power over a longer discharge cycle than the cellvolume alone would permit; of a process comprising: (a) dischargingelectrolytes 1 and/or 2, or fractions thereof, to such an extent thatwater will flow by osmosis from electrolyte 1 into electrolyte 2 whensaid electrolytes are separated by a water permeable membrane, and (b)circulating the discharged electrolytes 1 and 2, or fractions thereof,through the first and second chambers respectively of a container whichis divided by a water permeable membrane, under conditions wherein nocurrent flows in said container between electrolytes 1 and 2; for thepurpose of controlling the distribution of water between the twoelectrolytes.